ANZ customers received a notice from the bank explaining that changes were being made to the site.
However, the notice said the changes are cosmetic and services will not be affected.
Craig Reardon, director of e-commerce consultancy business The E Team made a complaint to ANZ at the start of the week requesting access to his online accounts. He says he is still awaiting a response.
Reardon, who holds three personal and two business accounts with ANZ, said all accounts have been affected.
He said most of the problems come after logging-in, when the service grinds to a halt.